The invention relates to a colour display tube comprising in an evacuated envelope a display screen and an electron gun system which comprises first means to generate three in-line electron beams. The first means comprise, for each electron beam, a cathode and a first and second electrode having apertures for each electron beam and being common for the three electron beams. The system further comprises second means to generate focusing lens fields for symmetrically focusing the electron beams on the display screen. The second means comprise at least two electrodes having apertures for each electron beam and being common for the three electron beams. The electron gun system produces asymmetric lens fields for the two outermost electron beams to deflect the two outermost electron beams toward the central electron beam to converge the electron beams on the display screen.
Such a colour display tube having a so-called integrated electron gun system in which a number of electrodes is constructed so as to be common for the electron beams is known from Netherlands patent application No. 7809160, corresponding to U.S. Pat. No. 4,291,251. In the embodiment shown in FIG. 6 of the above-mentioned patent application, the electron gun system comprises three cathodes, a common first and a common second electrode, with which three in-line electron beams are generated. Each of the electron beams is focused on the display screen by means of one single focusing lens field which is generated between the facing sides of a first and a second common focusing electrode.
In the first focusing electrode, the apertures for the two outermost electron beams on the side facing the second electrode are placed eccentrically with respect to the axes of the generated electron beams. As a result of this, an asymmetric lens field is formed between the facing sides of the first focusing electrode and the second electrode, which field deflects the two outermost electron beams in the direction of the central electron beam in such manner that the three electron beams converge on the display screen. The apertures in the two focusing electrodes at the area of the focusing lenses are staggered with respect to each other for the outermost two electron beams in such manner that symmetrical focusing lens fields are formed with respect to the outermost electron beams already deflected over the convergence angle. This has for its object that changes in the voltages of the focusing electrodes, and hence changes in the strength of the focusing lens fields, have no influence on the convergence of the electron beams.
Convergence of the electron beams independent of the focusing is of particular importance in those systems in which defects in static convergence are corrected by means of a ring of magnetic material placed in the neck of the display tube, which ring is permanently magnetized as a multipole from outside the tube dependent on the desired corrections. In such systems it is not possible to readjust the convergence of the electron beams from outside the tube to compensate for variations of the focusing voltages.
In the known tube convergence is effected by deflecting the two outermost electron beams toward the central electron beam near the apertures on the side of the first focusing electrode facing the second electrode. Deflecting the outermost electron beam in this spot has the disadvantage that variation in the voltages of the first focusing electrode and the second electrode cause small changes in the angle over which the outermost electron beams are deflected.
The changes in angle cause changes in the place of the virtual object which is displayed on the display screen by the focusing lens. These displacements result in beam displacements on the display screen and consequently convergence errors.